This invention relates to reconfigurable antenna systems, and more particularly, to an apparatus and method for reconfiguring antenna elements in a reconfigurable antenna array.
Reconfigurable antenna systems have applications in satellite and airborne communication node (ACN) systems where wide bandwidth is important and where the antenna aperture must be continually reconfigured for various functions. These antenna systems may comprise an array of individually reconfigurable antenna elements. An antenna array comprised of reconfigurable dipole elements can be reconfigured by varying the resonant length of one or more of the elements. The ability to dynamically vary the resonant length of a dipole antenna enables the antenna to be operated efficiently within multiple frequency ranges.
One means of varying the resonant length of a dipole antenna is to segment the antenna lengthwise on either side of its feed point. The resonant length of the antenna may then be varied by connecting or disconnecting successive pairs of adjacent dipole segments. Connection of a pair of adjacent dipole segments may be effected by coupling each segment to a switch. The adjacent segments are then joined by closing the switch.
Previous designs for reconfigurable antennas have been proposed which incorporate photoconductive switches as an integral part of an antenna element in an antenna array. See xe2x80x9cOptoelectronically Reconfigurable Monopole Antenna,xe2x80x9d J. L. Freeman, B. J. Lamberty, and G. S. Andrews, Electronics Letters, Vol. 28, No. 16, Jul. 30, 1992, pp. 1502-1503. Also, the possible use of photovoltaic activated switches in reconfigurable antennas has been explored. See C. K. Sun, R. Nguyen, C. T. Chang, and D. J. Albares, xe2x80x9cPhotovoltaic-FET For Optoelectronic RF/Microwave Switching,xe2x80x9d IEEE Trans. On Microwave Theory Tech., Vol. 44, No. 10, October 1996, pp. 1747-1750. One problem with these designs, however, is that the performance of ultra-broadband systems (i.e., systems having an operating frequency range of approximately 0-40 GHz) utilizing these types of switches suffers in terms of insertion loss and electrical isolation.
Radio frequency micro-electromechanical system (RF MEMS) switches have been proven to operate over the 0-40 GHz frequency range. A representative example of this type of switch is disclosed in Yao, U.S. Pat. No. 5,578,976. Previous designs for reconfigurable antennas using RF MEMS switches incorporated metal feed structures to apply an actuation voltage from the edge of a substrate to the RF MEMS switch bias pads. A problem with the use of metal feed structures to apply an actuation voltage to the switches is that, in an antenna array, the number of switches can grow to thousands, requiring a complex network of bias lines routed all around the switches. These bias lines can couple to the antenna radiation field and degrade the radiation pattern of the antenna array. Even when the bias lines are hidden behind a metallic ground plane, radiation pattern and bandwidth degradation can occur unless the feed lines and substrate feed through via conductors are very carefully designed because each element in the antenna array may accommodate tens of switches. This problem is magnified enormously as the number of reconfigurable elements increases. Thus, a need exists for an improved apparatus and method for actuating switches in a reconfigurable antenna array.
Accordingly, an object of the present invention is to provide a means for actuating RF MEMS switches without the need for metal feed structures coupled to the switches. Further objects and advantages of the invention will become apparent from a consideration of the drawings and following description.
The present invention uses a series of MEMS switches to reconfigure an antenna element in a reconfigurable antenna system. The MEMS switches and antenna element are mounted on a semi-insulating substrate. The MEMS switches are actuated by optical energy conveyed to the switches via an optical waveguide network integrated into a superstrate, which is coupled to the substrate. Preferably, the superstrate is radio frequency (RF) transparent. The RF transparent superstrate functions both as a framework for incorporation of the optical waveguide network and as a radome for the reconfigurable antenna system.